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Linear power supply design, efficiency

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Cobalt60

New Member
I, like many people starting to really get into electronics engineering as a hobby, intend to design and build my own variable (linear) bench supply. If Im not mistaken, pretty much all of the designs go something like this:

Transformer -> Rectifier -> Capacitor -> Regulator

Im assuming most people use regular transformers, so do most people just deal with the voltage drop being wasted as heat, or do people somehow minimize the voltage drop, and if so, how?

One thought I have is to use a variable transformer. Any other methods?

Thanks

-Chris P
 

Sceadwian

Banned
Variable transformers are bulky and expensive, but work really well a keeping the power down. There has been a suggestions on the forums to use a tap switch transformer to at least switch between using a full transformer or just half of a center tapped transformer, simple cheap and easy to implement. You could always just brute force it too, use a power mosfet as a current pass transistor for a linear regulator and just put the biggest hairiest heat sink you can find on it. I'm all for the brute force method, sure it wastes a lot of power but it works well if you can dissipate it and it's dead simple. This is just for a variable desk supply though for playing with things. You should have fixed supplies for common voltages (atx supplies work quiet well)
 

dknguyen

Well-Known Member
Most Helpful Member
*claps* he realizes that linear regulators burn off extra voltage as heat!

You can use a variable transformer or a DC-DC switching regulator to do that. Since it's AC-DC variable transformer like a variac might be simpler. Variacs are not isolated and you may want to add a 1:1 isolation transformer for that purpose.

You'll also need a variable resistor on your linear regulator so you can adjust the actual output voltage too, not just the input voltage to the linear regulator.
 
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Cobalt60

New Member
I tried to consider a switching reg, but havent really figured out how to smooth its output, plus I do have a concern with noise. And yes I know Id need a variable resistor to adjust the reg; the way my current bench supply is designed is it has a variac with a pot ganged to it for the reg.

So why design a supply if I already have a decent one? Come on, you all know the answer to that, dont you? Just to do it, and maybe I think I can get some better specs, not to mention make it a dual supply etc.

In some cases, I would like something with some really high amps, in that situation a switching ATX supply would be fine, but I also want a nice efficient supply for analog circuits also with some decent specs, good efficiemcy, and designed and built by me.

So any more ideas? Any other circuits to reduce (or divide) voltage while also being efficient?

Thanks

-Chris P
 

Cobalt60

New Member
Can a variac designed for a 240V input 0-280V output be used on a 120V input with a corresponding ratio (0-140V)? And would its amp rating remain the same?
 

dknguyen

Well-Known Member
Most Helpful Member
A variac designed for 240V just means that the insulation on the wires can tolerate that much so it should be okay to run it with a lower voltage. Amp rating remains the same due to wire size and core mass. THe ratios stay the same. Output voltage can be from 100% of input voltage all the way down to 0% of input voltage.
 
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mneary

New Member
A nit maybe, but potentially helpful: Most Variac® variable autotransformers that I see in the catalogs can be wired to be adjustable from 0 to 117% of the input. This can be good, if you want to check things above nominal line voltage, or it can create the risk of accidental overvoltage if you're careless.
 

Cobalt60

New Member
Ok so just to actually finish up this post... Basically if I want to minimize voltage drop, it would be done through the transformer. I could either use an adj transformer, or a center tap transformer (to achieve a half voltage output from it).

Other than that, theres no other "tricks" other than using switching, which I basically do not think I will be implementing.

Since you guys seem willing to take this just a bit OT, you can help me finish basically the other design aspect I should consider; whether or not I want this to be a bi-polar supply. First of all, I am under the impression that a bi-polar supplys positive voltage will equal to its negative voltage, they are not separately adjustable, is this true? Well, if I make this a dual supply (2 seperate uni-polar power supplies in one case), then I dont really need a bi-polar supply, right? Or would it still be useful to have a dual bi-polar supply in some cases?

So if you can just help me with that aspect, pretty much all of the questions I have will be in a separate post and will probably be after I started building it.

Thanks everyone!
 

Torben

Well-Known Member
Ok so just to actually finish up this post... Basically if I want to minimize voltage drop, it would be done through the transformer. I could either use an adj transformer, or a center tap transformer (to achieve a half voltage output from it).

Other than that, theres no other "tricks" other than using switching, which I basically do not think I will be implementing.
Pretty much. There are many different ways to "burn off" the extra voltage coming from the transformer but they all involve just that: converting the energy into some other form of energy that you can then get rid of. I learned this when making a 12VDC 1A regulated supply a couple of years back: all I had lying around was an 120V-18V transformer and I wanted to avoid having to put a freaking huge heatsink on the regulator. My choices were essentially A) put in some power diodes to drop the voltage (but the diodes then needed to dissipate the energy as heat), B) use resistors to drop the voltage (but then the resistors needed to dissipate the extra voltage as heat), C) put a heat sink on the regulator (and let it dissipate the heat), or D) use the proper transformer and not have to worry about it. :) I went with option D in the end.

(I'm sure there are other options as you noted, like switchers--but I'm also grateful that the people on forums like this recognized that I wanted to design it mostly myself and I didn't stand a hope in hell of designing a good, safe, and reliable switcher by myself at that point. Honestly I probably still don't).

Since you guys seem willing to take this just a bit OT, you can help me finish basically the other design aspect I should consider; whether or not I want this to be a bi-polar supply. First of all, I am under the impression that a bi-polar supplys positive voltage will equal to its negative voltage, they are not separately adjustable, is this true? Well, if I make this a dual supply (2 seperate uni-polar power supplies in one case), then I dont really need a bi-polar supply, right? Or would it still be useful to have a dual bi-polar supply in some cases?

So if you can just help me with that aspect, pretty much all of the questions I have will be in a separate post and will probably be after I started building it.

Thanks everyone!
Depends. You would need a dual supply for some analog circuitry but for digital stuff you should be fine in most cases with a single supply.

You could always design two supplies into one case using a common ground. One would be negative and the other positive relative to the ground. Then you could have either a single positive supply, or a single negative supply, or a dual supply, depending on the project.

But I'd start with building a decent single supply. Concentrate on making it work well, and on making it safe. You'll learn a lot doing this and can then apply that knowledge to the next one you build.

One thing I like to do with mine is put an LED between the rectifier and regulator to bleed off the filter caps when the thing is powered down. It's not usually necessary as an indicator (though it can be a cool one) since I usually use a lighted switch on the front panel, so I like to use a super-bright blue LED for this bleeder LED and just mount it somewhere inside the case. Then I get a nice blue glow from inside the thing when it's turned off, which fades out slowly after the device is switched off as it bleeds off the caps. :)


Cheers and good luck,

Torben
 
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Cobalt60

New Member
But I'd start with building a decent single supply. Concentrate on making it work well, and on making it safe. You'll learn a lot doing this and can then apply that knowledge to the next one you build.

One thing I like to do with mine is put an LED between the rectifier and regulator to bleed off the filter caps when the thing is powered down. It's not usually necessary as an indicator (though it can be a cool one)
Yeah I probably will start simple. I really like your idea using an LED to show when the caps are charged. I actually would like to have as many dials, indicator lights, switches, gauges, etc on the front panel of this thing as possible : ) (though the gauages I may have to limit based on cost). I was actually thinking of having jacks tapped directly to the transformer, so I would have an option to output AC. Not exactly sure how it will be set up, but I could either use this "discharge" LED to indicate that the supply is set to DC mode, or perhaps I could actually have a separate on/off switch for AC and DC, with the "discharge" LED indicating that the DC bus is powered.

Thanks
 
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